lab 8 - Preparation of Tertiary Alkyl Chlorides by...

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Preparation of Tertiary Alkyl Chlorides by Nucleophilic Aliphatic Substitution/Sodium Iodide and Silver Nitrate Tests for Alkyl Halides Mayank Kumar April 13, 2007 Methods and Background The purpose of the experiment was to demonstrate that a tertiary alcohol, 2-methyl-2-butanol, reacts with hydrochloric acid through unimolecular nucleophilic aliphatic substitution to form a tertiary alkyl chloride, 2-chloro-2-methylbutane; furthermore, sodium iodide and silver nitrate tests were used to confirm the presence of a tertiary alkyl chloride. Nucleophilic aliphatic substitution involves the substitution of one group for another at a saturated, sp 3 -hybridized carbon atom. In nucleophilic aliphatic substitution, a nucleophile, a neutral molecule or anion that has at least one nonbonding pair of electrons, and thus Lewis base character, attacks an electrophilic, Lewis acidic, carbon atom, donating its nonbonding pair of electrons to form a new covalent bond, while the leaving group, which is neutral or negatively charged, departs as it accepts the pair of bonding electrons from the carbon atom as the bond between it and the carbon atom breaks. The reaction can be considered to be a Lewis acid-base reaction because the electronegative leaving group imparts Lewis acidic character on the carbon atom to which it is attached due to polarization of the bond. One mechanism of nucleophilic aliphatic substitution is unimolecular nucleophilic substitution (S N 1). In this mechanism, the bond between the carbon atom and the leaving group undergoes heterolytic cleavage, or ionization, utilizing the assistance of the polar interactions between solvent molecules and the incipient cationic and anionic centers, to yield a carbocation, an intermediate with a positively charged carbon atom (the carbon atom that was bonded to the leaving group). The carbocation then combines with a nucleophile to form the substitution product. The first step of an S N 1 reaction, formation of a carbocation, is much slower than the second step because it involves breaking the bond between the carbon atom and the leaving group to generate an unstable carbocation, which is an endothermic process; therefore, it is the rate-determining step of the reaction. In contrast, the second step, nucleophilic attack of the carbocation to yield the product, is much faster because it is an exothermic process. Since the first step of the reaction is the rate-determining step, the rate of the reaction depends only upon the concentration
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of the substrate that is undergoing substitution: Rate = k 1 [R-L], where k 1 is the first-order rate constant and R-L is the substrate undergoing substitution, with L representing the leaving group. The other mechanism of nucleophilic aliphatic substitution is bimolecular nucleophilic
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This lab report was uploaded on 04/07/2008 for the course CHEM 233 taught by Professor Landrie during the Spring '08 term at Ill. Chicago.

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lab 8 - Preparation of Tertiary Alkyl Chlorides by...

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